Digital cameras and other digital imaging systems are becoming widely used in both consumer and industrial applications. In remote, hard to reach and/or dangerous environments like outer space, high radiation areas, mines and military action zones, these devices offer an important ability to obtain images at low cost and risk. However, these same locations may impart much more stress on the imaging system; for example, the imaging system may be subject to increased radiation, heat and/or pressure. These types of stresses can lead to partial failures of the imaging system, especially in the image sensor elements, which occupy a significant percentage of the circuit area. At the same time, deployment of digital imaging systems in these types of areas makes it difficult or impossible to replace failed devices. In addition, enhanced reliability and reduced manufacturing cost are important for consumer applications. Hence, there is considerable advantage to creating self-repairing and correcting imaging system, that remove defects which may arise either during fabrication or during the extended lifetime of the system.
Digital imaging systems, such as digital cameras, comprise a matrix of optical sensing picture elements or “pixels”. The number of pixels in the sensing area determines resolution of a captured image. However, the more pixels, the higher the probability that at least some of the pixels will fail, either during fabrication or when the pixels are later exposed to a stressful environment. Indeed, many digital cameras already employ software correction techniques to reduce the effect of errors caused by failed pixels. Such techniques involve replacing the values of failed pixels with a weighted average of light from adjacent pixels. However, such software interpolation inherently cannot give the correct value under all conditions, such as when the light intensity is changing rapidly across the image.
Furthermore, there is considerable interest in creating digital imaging systems having large area imaging sensors. For example, an imaging sensor which could cover the standard film area of a 35 mm camera would allow existing analog camera lenses to be used with a digital camera and would provide much greater resolution than current digital cameras.
Typical image sensors used in digital cameras comprise array of charged coupled devices (CCDs). From the sensor production point of view, it is difficult to fabricate error free CCD arrays on the order 36×24 mm (i.e. the standard 35 mm film frame size). Producing a CCD array of 35 mm size or larger has been done, but is very expensive. In addition, production of sensors of that size usually results in CCD arrays containing one or more defective pixels and, therefore, a correspondingly low production yield.
CCD arrays are very sensitive susceptible to failures. Since pixel data is typically passed down columns though other pixels, single pixel failures can lead to loss of whole columns of image data. CCD arrays typically require nonstandard CMOS fabrication, which tends to further increase production costs and reduce production yields. CCD arrays also typically require multiple voltages on chip for efficient charge transfer. CCD arrays having a large number of pixels also tend to have relatively slow output and high power consumption.
Active pixel sensors (APS) have recently emerged as a competitor to CCD technology in the digital imaging field. APS arrays combine arrays of photodiodes (or photogates) with some selection circuitry. APS devices have the advantage that they may be fabricated using standard CMOS fabrication technology which makes them cheaper to fabricate. While not as optically sensitive as CCD devices, APS devices may be operated using only a single voltage and can typically consume less power than CCD devices. Also because APS devices are fabricated using standard CMOS fabrication technology, they can integrate A/D converters and control circuitry on the same chip, thus lowering system costs.
In U.S. Pat. No. 5,471,515, Fossum et al describe an active pixel sensor comprising a photodiode together with readout, row-select and reset transistors. Light hitting the photodiode creates current, which charges the photodiode and the gate of the readout transistor, thus creating a gate voltage that is proportional to the illumination intensity-exposure time product. In U.S. Pat. No. 5,608,243, Chi and Bergemont disclose a variation on the standard APS cell in which the photodiode is replaced with a split gate MOS transistor with one transistor forming the detector, and the other the readout transistor. In U.S. Pat. No. 4,760,458, Bergemont and Chi disclose an APS cell which includes bipolar phototransistors.
In U.S. Pat. No. 6,043,478, Wang et al disclose an APS having a shared readout structure, where two photodiodes and their readout transistors feed into a single row-select transistor, thus cutting the number of readout columns in half.
It is know, in general, to provide a system which includes redundant elements. However, in such systems the redundant elements are not used by the system unless there is a failed element that is being replaced, hence such ideal spares add area to the system without providing useful operations unless a failure occurs.